The present invention relates to a tandem pneumatic brake booster for a vehicle. The object of the invention is to allow a return of a brake pedal to the rest position after a maximum stroke of a thrust rod of this booster. The invention prevents the retention of a vacuum in a first front chamber of the booster. The invention relates more particularly to the field of automobiles but may also apply in other fields.
A tandem pneumatic brake booster for a vehicle is situated between a brake pedal of the vehicle and a master cylinder. The booster makes it possible to convert a pressure of a driver's foot on a brake pedal into a pneumatically amplified pressure. This pneumatically amplified pressure is designed to brake the vehicle. Accordingly, the booster makes it possible to markedly lessen a force supplied by the driver when he presses on the brake pedal.
Such a booster comprises, in a booster shell, a front chamber and a first rear chamber mounted in series with a second front chamber and a second rear chamber. The first front chamber and the second front chamber are respectively closer to the master brake cylinder than the first rear chamber and the second rear chamber. The second front chamber and the second rear chamber are closer to the master cylinder than the first front chamber and the first rear chamber.
Each of the chambers comprises a variable volume. The first and the second front chamber are respectively separated in a sealed manner from the first and second rear chamber by a first and second plate skirt. Each plate skirt is formed by a sealed and flexible membrane associated with a rigid plate. The membrane of the plate skirt rests on the rigid plate. The rigid plate is pressing on a pneumatic piston forming the thrust rod which actuates the master cylinder. The membrane is attached to the pneumatic piston on one side, and to a wall of the booster shell opposite the pneumatic piston on another side.
The first front chamber is separated in a sealed manner from the second rear chamber by a fixed rigid partition in the booster. This fixed partition is fixedly attached to a wall of the shell opposite the pneumatic piston.
The first and the second front chambers communicate with one another via one or more communication ducts. Such communication ducts open into a central hollow portion of the pneumatic piston. The number of ducts present in the pneumatic piston must be sufficient to allow a good communication between the first front chamber and the hollow central portion of the pneumatic piston, without weakening the structure of the pneumatic piston. Such communication ducts between the hollow central portion of the piston and the first front chamber have one end opening into the first front chamber, at a location close to a bearing surface of the first plate skirt. They have one end opening into the hollow central portion of the pneumatic piston. The hollow central portion of the pneumatic piston opposite the master cylinder is open and communicates with the second front chamber.
One of the front chambers, usually the second front chamber, is connected to a vacuum source. This vacuum source may, for example, be a vacuum pump fixed to the shell of the booster. This vacuum pump then communicates with the front chambers. The vacuum pump maintains a vacuum in the two front chambers thanks to the communication ducts between them.
The booster furthermore contains a communication valve. This valve blocks communication between the front chambers and the rear chambers and communication between the rear chambers and the atmosphere depending on the position of the pneumatic piston in the booster.
In the rest position, that is to say when the pneumatic piston is not moved forward by the booster, the front chambers communicate freely with the rear chambers. The chambers are then all isolated from the atmosphere. The vacuum that is imposed by the vacuum pump is then established in the front chambers and in the rear chambers.
At the time of braking, during a first phase of movement of the pneumatic piston in the booster, communication between the front and rear chambers is cut off by a communication valve. The front chambers are then isolated from the rear chambers.
During a second phase of piston movement, the rear chambers are placed in communication with the atmosphere. When the rear chambers are in communication with the atmosphere, the front chambers are still isolated from the rear chambers and still contain a vacuum.
The communication of the rear chambers with the atmosphere causes the pressure to rise in the rear chambers to the same level as the atmospheric pressure. There is then a pressure difference between the front chambers and the rear chambers. This pressure difference moves forward the plate skirts separating the front chambers from the rear chambers and moves the pneumatic piston on which the plate skirts rest.
A return spring is usually placed inside such a booster. The function of this return spring is to help the pneumatic piston to return to the rest position after braking. Such a spring presses on the wall of the booster close to the master cylinder on one side and on the pneumatic piston on the other side.
Once the brake pedal is no longer pressed, the return spring pushes the pneumatic piston back to its rest position. The rear chambers are then no longer in communication with the atmosphere. Communication between the rear chambers and the front chambers is then reestablished. The vacuum present in the front chambers then extends into the rear chambers.
In order to seal the rigid partition separating the first front chamber from the second rear chamber, without hampering the movement of the pneumatic piston in the booster, a seal is situated on an inner snout of the fixed rigid partition. This seal completely surrounds the pneumatic piston in a sealed manner. During its movement, the pneumatic piston slides in a sealed manner inside this seal. The sealing of the seal prevents the vacuum in the front chambers from disappearing when the pressure in the rear chambers rises.
Such a seal comprises a first lip. This first lip extends parallel to the axis of movement of the pneumatic piston. This first lip extends in a direction opposite to the direction of travel of the pneumatic piston during braking. This first lip is pressed against the wall of the pneumatic piston in a sealed manner. This first lip is housed in the first front chamber. This first lip provides the seal between the first front chamber and the second rear chamber.
During braking, a master cylinder activated by a thrust rod must always carry out a maximum stroke of movement that is less than the stroke of movement of the pneumatic piston of the booster. In this manner, the thrust rod comes to a stop in the master cylinder before the plate skirt of the first chamber comes into contact with the fixed rigid partition. Accordingly, the maximum stroke in the master cylinder must be less than the maximum stroke of a thrust rod. When the master cylinder and the pneumatic piston of the booster have very similar stroke lengths, it may be, particularly for reasons of manufacturing tolerances or of deformation clearances of the parts of the booster due to the pressure, that the first plate skirt comes into contact against the seal fixed to the fixed rigid partition.
This contact between the first plate skirt and the seal occurs on the edge of the first lip pressed against the wall of the pneumatic piston. If necessary, this contact occurs on the wall of a second lip facing the first plate skirt. This contact causes the first plate skirt to adhere to the seal. When such an adhesion occurs, there is no longer communication between the two front chambers. Specifically, the seal, adhering to the plate skirt, inserts itself in a sealed manner between the first front chamber and the communication duct between the front chambers that are present on the pneumatic piston.
The vacuum in the first front chamber is then maintained inside the first front chamber. Then a suction effect is created which prevents the first plate skirt from separating from the seal when the pressure on the pneumatic piston is relaxed. The first plate skirt being fixed to the pneumatic piston, the return to the rest position of the pneumatic piston inside the booster cannot take place. The result of this is that the brake pedal no longer returns to its position. The vehicle remains immobile or, at best, releases slowly.
To solve this problem, a more powerful return spring could be used. However, such a spring, having sufficient power to release the first lip from the seal of the plate skirt despite the vacuum, would reduce the effectiveness of the thrust of the plate skirts due to the pressure difference between the chambers. The plate skirt would then no longer move the pneumatic piston in a satisfactory manner to allow the booster to correctly fulfill its role.
Another solution that can be imagined has consisted in adding protrusions to the wall of the second lip of the seal fixed to the fixed partition opposite the first plate skirt. These protrusions were to make it possible to create a circulation space to release the vacuum from the first front chamber. This space was thought to prevent the vacuum from persisting in the first front chamber. However, such a solution is revealed by experience to be ineffective. Specifically, the deformation of the seal and the first plate skirt by the pressure does not prevent the vacuum from being trapped in the first front chamber. This solution does not provide sufficient communication between the front chambers.
To solve this problem, the invention makes provision to place protrusions on the edge of the first lip of the seal opposite the first plate skirt, preferably while associating these protrusions on these edges with protrusions situated on the second lip of the seal. The first lip of the seal then has the shape of a crenelated lip.
When the first plate skirt moves towards the fixed partition separating the first front chamber from the second rear chamber, the first plate skirt presses on the tops of the merlons of the first lip of the seal.
A communication leakage is then created still in the space between the merlons of the first lip of the seal. The plate skirt cannot adhere in a sealed manner to the seal in this space. In the same manner, this communication leakage may be extended between protrusions of the second lip of the seal. The vacuum may then be released by these communication leakages and no longer remains blocked in the first front chamber. The suction effect that this vacuum created no longer exists, and the first plate skirt and the pneumatic piston can then return to the rest position.
The subject of the invention is therefore a tandem pneumatic brake booster for a vehicle comprising a shell, a pneumatic piston sliding in the shell, two pairs of chambers, each pair comprising a front chamber and a rear chamber, the front chamber being situated closer to a brake master cylinder than the rear chamber, a fixed partition in the booster, separating a first front chamber from a second rear chamber, a first plate skirt, separating the first front chamber from the first rear chamber, and a second plate skirt, separating the second front chamber from the second rear chamber, these plate skirts being movable, sealed and moving the pneumatic piston, a seal fixed to the fixed partition, the pneumatic piston sliding inside this seal in a sealed manner, this seal comprising a first lip pressed on the pneumatic piston in a sealed manner, and extending parallel to the axis of movement of the pneumatic piston, characterized in that the first lip is crenelated.